fix(mcp): close MCP client session after listTools/callTool to fix Windows libuv assertion and process leak

[cstavaru]

Summary

MCPToolset.getTools() and MCPTool.runAsync() open a new MCP client
session via mcpSessionManager.createSession() but never call
session.close() afterwards.

For stdio transports — which is the most common configuration — this
leaves the spawned MCP-server child process and its stdio pipes alive
as libuv handles for the rest of the parent process's lifetime, with
two visible consequences:

1. The Node event loop never drains. Any program that uses
   MCPToolset cannot exit naturally after its work is done; callers
   are forced to process.exit().

2. Windows libuv assertion crash on process.exit(). When the
   parent eventually does call process.exit(0), libuv races to tear
   down the still-open async handles for the stdio pipes and aborts:

   Assertion failed: !(handle->flags & UV_HANDLE_CLOSING),
   file src\win\async.c, line 76
   

   This is reproducible on Windows whenever the parent program calls
   process.exit() while an MCPToolset's stdio transport is still
   alive.

3. Per-invocation child process leak. Every runAsync call leaks
   one child process for the lifetime of the parent.

Fix

Add await session.close() after the listTools() call in
MCPToolset.getTools() and after the callTool() call in
MCPTool.runAsync(). This releases the underlying transport (stdio
child or HTTP connection) immediately, so the event loop drains
naturally and there is nothing left for libuv to tear down at exit.

The diff is two lines:

   const session = await this.mcpSessionManager.createSession();
   const listResult = (await session.listTools()) as ListToolsResult;
+  await session.close();
   logger.debug(`number of tools: ${listResult.tools.length}`);

   const result = await session.callTool(callRequest.params, undefined, {
     signal: request.toolContext.abortSignal,
   });
+  await session.close();
   return result as CallToolResult;

Verification

Locally:

• npm run lint — clean
• npm run build — clean
• Reproduced the libuv assertion on Windows before the fix using a
  small CLI that calls MCPToolset.getTools() followed by
  process.exit(0); assertion no longer fires after the fix and the
  process exits cleanly without an explicit process.exit() call.

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[kalenkevich]

I think we need to update MCPSessionManager to support close method as well.